Murray McEwan, a chemist at the University of Canterbury who works with
NASA on planetary atmospheres, responded.

Air is the "stuff" that makes up the Earth's
atmosphere. Our Earth is just one of the planets that make up our Solar
system of Sun and planets and most, but not all of these planets have
atmospheres. Earth is special because it's where we live and its
atmosphere is special because it is very different from the atmospheres
of all the other planets.

So where did the Earth's atmosphere or "air" come from? Scientists are
unsure what the Earth's atmosphere was like billions of years ago. The
most common elements in the Universe are hydrogen and helium and whereas
there is a lot of hydrogen and helium in the atmospheres of the outer
planets like Jupiter and Saturn, the Earth's atmosphere is very different.
Hydrogen and helium are light gases and they escaped very early in the
Earth's history. Eventually they were replaced by carbon dioxide, some
nitrogen, and water coming out of the earth from volcanic eruptions
along with smaller amounts of sulphur oxides and very small amounts of
ammonia and methane.

But the atmosphere is very different today than it was then. Today
the ingredient that makes our atmosphere different from all the other
planets is oxygen. It is a rather large proportion of air (about 21 per cent).
Initially oxygen may have been formed in small amounts by sunlight breaking
up water molecules (H2O) in the atmosphere. Today most of the oxygen comes
from photosynthesis driven by the engine of life that converts water into
oxygen. If there was no life on the Earth, then we estimate that the
amount of oxygen in our air would be billions of times less than what
it is today.